When a car collides with an oncoming car or another object, the driver may smash his face strongly onto the steering wheel or the front glass. If the impact generated at that time can be reduced, it is possible to prevent death or serious injury to the driver.
In many countries, drivers are obligated by law to use seat belts as one means of safety upon collisions, but the effective reduction of the impact by seat belts is not sufficient when a car collides with an object at a high speed.
Therefore, use of an air bag system, i.e., a system capable of sensing an impact caused when a car collides with an object and instantaneously inflating a bag housed in a center portion of a steering wheel or the like to ensure the safety of the driver or the like is now being considered in many countries as a more reliable safety measure.
One conventional air bag has been manufactured by forming a polymer layer on a plain woven fabric, providing a gas introducing hole in one of the woven fabrics, superimposing two woven fabrics with polymer layers in a state with the polymer layer sides facing each other, and sewing together circumferential edges of the two woven fabric. For example, Japanese Unexamined Patent Publication No. 51-4742, in the portion explaining the conventional air bags, describes that conventional air bags are obtained by cutting two circular pieces of woven fabrics from two regular square pieces of woven fabrics and sewing together the circumferential edges of the two circular pieces.
In air bags, a gas is instantaneously fed into the bag by explosion of a pyrotechnic, so the air bag must have a strength sufficient to endure the impact force caused by the explosion. The above-mentioned method of manufacturing air bags using a sewing process has several problems in this regard. There are many manual working processes involved, so there is a chance of decreased strength of the sewn portions and inspections of the strength of the sewn portions and the air permeability of the woven fabric take much time. The overall reliability of air bags manufactured by this method is low.
A method of manufacturing air bags using a tubular weaving method has been proposed. Japanese Examined Utility Model Publication No. 57-58228, corresponding to Japanese Unexamined Utility Model Publication No. 49-56352, discloses an impact absorbing tubular woven fabric having bag portions formed by alternately folding two woven up and down at a middle portion. In this tubular weave, a first bag portion and a second bag portion having a connected portion, which consists of the upper woven fabric and lower woven fabric woven together in a jointed weave, are alternately provided. The second bag portion is smaller than the first bag portion. The fact that this tubular woven fabric could be coated was mentioned in this patent publication, but a detailed explanation was only given of a continuous body of an air bag.
Further, Japanese Examined Patent Publication No. 54-576, corresponding to Japanese Unexamined Patent Publication No. 50-7232, disclose an air bag having an upper connected portion and a lower connected portion having weft threads with larger shrinkage ratios than their warp threads. In the description of the invention and the drawings of this publication, there was shown an air bag having a square shape in plan view, provided at the outer layer thereof with a covering layer.
However, when making separate bags from the impact absorbing tubular woven fabric disclosed in Japanese Examined Utility Model Publication No. 57-58228, the above tubular woven fabric should be cut along the widthwise direction in a portion, so the shape of the cut fabric necessarily becomes square. An air bag having a square shape has problems such that the air pressure is not applied uniformly over the air bag, the appearance of the air bag is inferior, and it is difficult to neatly house the air bag in the steering wheel. The air bag disclosed in Japanese Examined Patent Publication No. 54-576 also has a square shape, so it has the same problems.
It is possible to make the chamber into which air is blown an octagon by sewing the four corners of an air bag of a square shape (in this case, the appearance of the air bag is still square), but this sewn air bag has disadvantages of the large cost of the labor to sew the corners, the weaker strength of the sewn portions compared to the other portions, and the lack of uniformity. Even if this air bag is turned inside out by pulling the inside of the air bag through the opening at the center portion of the air bag for the air insertion, only the appearance of the air bag is improved. The other disadvantages, such as the inferior uniformity, are not eliminated.
An object of the present invention is to provide an impact absorbing bag having superior uniformity, appearance, and reliability and having the ability to be easily manufactured.